Transimpedance amplifiers are frequently used for preamplification of opto-electronic signals from photodiodes. Transimpedance amplifiers are constructed of operational amplifiers or of discrete components (bipolar or FET). Here, circuits of discrete components (bipolar or FET) are preferred for higher frequencies and low noise over circuits made from operational amplifiers.
In all photodiode preamplifiers, and particularly in transimpedance amplifiers, the high dynamics which the optical signal hitting the photodiode may have represent a considerable problem. The power of the incident optical signal can be between 10 nW and several mW depending on the distance from the transmitter or the type of optical coupling. The dynamics of the photocurrents generated in the photodiode and forming the input signal of the preamplifier are correspondingly high. Since preamplifiers are optimized for very low input currents and have limited dynamics, preamplifiers are overloaded relatively quickly by the photocurrents. This considerably affexts or even nullifies the function of preamplifiers.
FIG. 1 shows a solution for a transimpedance amplifier with overload protection. In the transimpedance amplifier according to FIG. 1 a diode D is connected parallel to the feedback resistor RF. If the voltage drop from the resistor RF is greater that approx. 0.5 V, the diode D switches on and bridges the resistor RF with the substantially smaller diode flow resistance. Since the resistor RF determines the amplification (transimpedance), amplification is thereby decreased and overload is avoided. In this way it is possible to achieve an overload protection with a factor of 30 to 50.
Drawbacks of the arrangement according to FIG. 1 are the relatively low bandwidth, the signal distortions occurring with large signals and high frequencies, and the relatively low overload factor achievable.
Since the overload protection only acts on the feedback resistor (RF) and not on the load resistor Rc, the voltage drop from Rc rises very quickly with the collector current of Q1, so that transistor Q1 becomes saturated. Saturation generates delays and distortions in the signal, i.e. the overload protection is practically nullified and limited to low optical power values. Further drawbacks arise from the protective diode D1 itself that is used. Diode D1 corresponds to a capacitance which is parallel to resistor RF and which considerably reduces the bandwidth of the amplifier. In addition, the entire capacitance is strongly reversecharged, particularly in the case of pulse-like signals and overload, by the changing voltage from RF, so generating unwelcome snap-off effects and additional distortion of the signal.